In an interview with The Verge Marc Whitten (Xbox Live VP) confirmed that Xbox One will not feature Xbox 360 backwards compatibility.
This was to be expected with the switch to a different Hardware Architecture making it somewhat difficult – the same problem suffered by the PS4.
Speaking to The Verge he said:
The system is based on a different core architecture, so back-compat doesn’t really work from that perspective.
We’ll have more on the Xbox One as we get it.